This invention relates generally to the field of manufacturing semiconductor integrated circuit devices and more particularly, it relate to a method of electrically measuring a thin oxide thickness by tunnel voltage in a semiconductor integrated circuit device on a more efficient and effective basis.
As is generally well-known to those skilled in the art of manufacturing wafer-scale semiconductor integrated circuit devices and in particular those related with flash EEPROM technologies, there is required in the fabrication process of the flash memory cell a critical thin dielectric in the form of a tunnel oxide for "F-N" (Fowler-Nordheim) tunneling. This tunnel oxide must be thin (i.e., approximately 100 .ANG.) for F-N tunneling to provide erasing. Therefore, the precise control of the tunnel oxide thickness is essential during the fabrication of the flash memory cell. Further, as integration level increases, other technologies (such as general logic) also require thinner and thinner oxide (&lt;100 .ANG.) as gate oxide. Thus, the thickness control of the oxides in those applications has become equally important.
There is already known heretofore a technique for measuring optically the tunnel oxide thickness on test wafers after oxidation. However, as technology demands thinner and thinner oxides, the optical measurement of the oxide thickness on the test wafer becomes unreliable since this technique is generally accurate for measuring of thicknesses down to approximately 100 .ANG.. Further, the thickness measurement performed on the test wafers may or may not be representative of the actual or true thickness on the individual product wafers due to a number of varying factors such as the furnace positions of the test wafers with respect to the individual product wafers, the conditions of the test wafers, the calibrations of the metrologies, the doping concentration of substrate of the product wafers, and the like. More importantly, it is the "electrical" oxide thickness that determines the performance of the devices and not necessarily the thickness measured optically. Therefore, there is a strong desire to know the oxide thickness electrically on the actual product wafers. In order to overcome these possible variations, a traditional method of measuring the oxide thickness is performed on the individual product wafers at the end of the wafer fabrication process which is achieved by a conventional capacitance measurement technique.
However, one major drawback of these capacitance measurement techniques is that they require relatively complex electronic circuits to perform the capacitance measurements, thereby resulting in increased manufacturing costs. In one such capacitance measurement technique, there is used a feedback charge method as disclosed in "Package 82 Simultaneous CV Instruction Manual," by Keithley Instruments, Inc., Cleveland, Ohio, 1987. This feedback charge method for making the capacitance measurements include a feedback charge amplifier functioning as an integrator and a feedback capacitor connected between the inverting input and the output of the amplifier. A switch is connected in parallel with the feedback capacitor. One terminal of the unknown capacitance is connected to a voltage source, and the other terminal end thereof is connected to the inverting input of the amplifier. In use, the voltage source applies a step voltage, and the charge on the feedback capacitor is determined by measuring the output of the integrator both before and after the step voltage. The unknown capacitance is then calculated through a known equation. As a result, it can be seen that such capacitance measurements cannot be easily incorporated into an automatic tester of the type similar to Model S900A Parametric Tester which is commercially available from Keithley Instruments, Inc. Therefore, such capacitance measurements must frequently be performed manually. Consequently, the tunnel oxide thickness on the individual product wafers are checked quite infrequently and then only on a few of the wafers.
It would therefore be desirable to provide an improved, convenient method of electrically measuring efficiently and effectively a thin oxide thickness by tunnel voltage on a product wafer on a lot-by-lot, wafer-by-wafer and site-by-site basis in a semiconductor manufacturing environment in order to monitor and detect both systematic and abnormal changes. Further, it would be expedient that the electrical measurement technique be easily incorporated into an automatic tester for fast and high volume data collection.